Conventionally, conductive pastes (conductive resin compositions such as conductive ink or conductive paint and conductive adhesive) have been used widely for preparation of electrodes and conductive circuit patterns on printed wiring board substrates. Recently, the conductive pastes are used in preparation of printed wiring boards, IC tags, and circuit patterns for electromagnetic shielding. Such a conductive paste is a dispersion of conductive powder metal particles dispersed, for example, in a resin component or an organic solvent. Recently, there is increasing need for reduction in size not only of the circuit patterns on printed wiring substrates but also of the circuit patterns for IC tags or electromagnetic shielding materials. However, it is becoming increasingly difficult to cope with the demand for miniaturization of the circuit pattern with conductive pastes containing conventional metal particles having a size in the micron order, and thus, fine metal particles having a size in the nano order are attracting attention for production of accurate conductive pattern.
There are various old and new reports on a fine metal particle dispersions, and most of them are hydrosols, and the concentration of the fine metal particles therein was low. For use in applications such as conductive paste, the dispersion is advantageously organosol, and for exhibition of the properties as conductive material, the concentration of the fine metal particles is preferably higher, and thus, there exists a need for a high-yield method for producing a higher-concentration organosol.
The production methods for fine metal particles include gas-phase methods of producing particles in gas phase by vaporization and liquid-phase methods of reducing a metal compound in liquid phase by using a reducing agent such as ultrasonic wave, ultraviolet ray, or a reducing agent (see for example Patent Documents 1 and 2 below). The gas-phase methods, which are commonly carried out in pure gas, give less contaminated fine metal particles but often demand a special large-scale apparatus, and thus, the liquid-phase methods are advantageous, from the viewpoints of cost and convenience during preparative operation. Among the liquid-phase methods, methods of reducing fine metal particles by using a reducing agent are popular, and the reducing agents favorably used include hydrogen, diborane, alkali metal borohydride salts, quaternary ammonium borohydride salts, hydrazine, citric acid, alcohols, ascorbic acid, and amine compounds.
However, the reducing agents such as diborane, alkali-metal borohydride salts, quaternary ammonium borohydride salts, hydrazine, and others are very strong reagents leading to vigorous reaction with a metal compound, and thus, it is difficult to control the reaction rate, which leads to aggregation and precipitation of the fine metal particles generated, and thus it is difficult to obtain a favorable fine metal particle dispersion at good yield. In addition, many of the highly reductive reducing agents are highly basic and toxic, and thus, hazardous during operation. Reaction with citric acid, ascorbic acid, or an alcohol demands a high temperature condition such as refluxing during reduction, which generates fine metal particles at thermally elevated energy that are unstable and easily cause aggregation, and thus, it was difficult to raise the concentration of the particles. Although there are some reports on reduction by using an alcohol amine leading to relatively mild progress of reaction and subsequent extraction of the generated fine metal particles from the aqueous phase to the organic phase, it was not possible to obtain a particle dispersion containing fine metal particles with a smaller particle size distribution at high yield even by these methods. Unfavorably, some of amine compounds only form an amine complex and do not initiate reductive reaction, depending on the metal species, and thus, the kinds of the metals used are limited. In other words, the amine compounds are less flexible in practical use. It was difficult to obtain a uniform high-concentration fine metal particle dispersion with any conventional reducing agent, and there exists a need for development of a reducing agent that is safe and more reductive, giving fine metal particles having a smaller particle size distribution.
In forming a circuit with a conductive paste containing conventional micron-size metal powder, it was needed to sinter it at a high temperature of 400° C. or higher after formation of the circuit, thus imposing restriction on the substrate to be used, and it was necessary to sinter the circuit formed on a low melting base material such as plastic film at a temperature of 150 to 200° C. or lower, more preferably 150° C. or lower. Generally, when the average particle diameter of the fine metal particles is several nanometers (nm) to dozens of nm, the rate of activated-energy atoms present on the particle surface increases with respect to all atoms forming the particle, and extremely increased diffusion of metal atoms onto the surface leads to drop of the sintering temperature to a temperature significantly lower than the melting point of the bulk metal. Recently, the method for producing a conductive film by sintering fine metal particles at low temperature by utilizing such a phenomenon is attracting attention, and various methods for producing fine metal particles were proposed.
An example thereof is a method for producing fine metal particles by using a high-molecular weight pigment dispersant as protecting agent, adding a nonaqueous solvent and water thereto, reducing a metal compound with an amine, and extracting the reduced metal into the nonaqueous solvent (see for example Patent Document 3 below), but the method employed a high-molecular weight pigment dispersant having a high thermal decomposition temperature and thus, prohibited sintering at low temperature. There is also known a method for adding an amine compound such as octylamine as protecting agent and also an acidic decomposer during production by the gas-phase method and thus, to decrease the thermal decomposition temperature of the protecting agent, but, in preparing a circuit, it was needed to heat at 200° C. or higher for a long period of time and thus difficult to form the circuit of a plastic film even by the method. In addition, the method is also disadvantageous economically, because it is a gas-phase method.
There are recently also reports on methods of forming a circuit by adding a short fatty acid metal compound or an amine metal complex having a smaller number of carbon atoms as the protecting agent for fine metal particles and sintering the dispersion at low temperature (see for example Patent Document 4 below), but the fatty acid metal compound and the amine metal complex are highly reactive and easily reduced, causing problems such as progress of reductive reaction by light and heat during long-term storage, growth or aggregation of fine metal particles, and deterioration in stability of the dispersion. In addition, because the reducing agent used by the method is a very strong reducing agent such as t-butylamine borane, sodium borohydride, or hydrogen gas, the reaction rate is so fast that it is difficult to control the particle growth. Accordingly, fatty acids having 10 or less carbon atoms, in particular, generate a greater amount of aggregates, prohibiting production of a favorable dispersion.
It was recently found that a thin film having a volume resistivity in the 10−6 Ω·cm order could be obtained by using fine metal particles having a particle size in the nano order (see for example Patent Document 5 below), but unfavorably, it was necessary to sinter the film at a high temperature of 200° C. or more to make it sufficiently conductive and achieve a desired resistivity. Silver is unfavorably sensitive to and easily sulfidized by sulfur atoms present in air such as SO2 gas, and exposure of the circuit pattern face to ambient air results in generation of sulfide film and associated discoloration and deterioration in conductivity, or deterioration in communicative properties when used as an antenna circuit, and thus, there existed a need for a method for preventing metal sulfidization.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 11-80647
Patent Document 2: JP-A No. 61-276907
Patent Document 3: JP-A No. 2002-334618
Patent Document 4: JP-A No. 2005-81501
Patent Document 5: JP-A No. 2005-120226